Home > MacroLetter > 7.1 - Autumn 2001 > Winding Options - Coex Barrier Films

Winding Options for Coextruded Barrier Films

Barrier film lines manufactured today are designed to produce from 5 to 9 or more discreet layers in a single film structure.  The versatility demanded from these film lines means that they are required to produce films that vary from tacky to slippery, extensible to stiff, multiple in-line slit rolls or one up, thick films/low line speed or thin films/high speed.  These varying operating conditions must be considered when choosing the film winders to ensure they are capable of winding this broad range of products effectively. 

There are two options available for winding coextruded barrier films: a center turret winder, or a surface winder with center assist/gap winding capability.  Each of these winder styles should offer two-direction winding capability to afford maximum flexibility when winding asymmetrical structures or winding treat side in or out.  Tension setting and control are critical in determining film and roll quality.  It is essential that constant tension upstream operations such as slitting be performed in separate isolated tension zones from the winding process.  This isolation is achieved with a pull roll; either a nip or an “S” wrap.  Isolating the tension during winding allows the precise control of tension in the wound roll and also permits the use of taper tension, a feature that gives the ability to reduce the tension level as the roll diameter builds, reducing roll conformation defects.   Center-seeking dancers or load cells in concert with a sophisticated variable speed drive controller (AC or DC) are required to provide accurate, consistent tension control.  These dancers or load cells can be set up to control the speed or more commonly are programmed to trim a speed reference signal from an upstream drive by 10% to 15%.  Load cells offer the benefits of a simpler threading path as well as providing an accurate readout of actual tension values versus dancers, which are calibrated on a pressure regulator.  However, dancers provide a small amount of accumulation that will compensate for changes in web tension and distance of travel, which occur during the indexing cycle on a turret winder.

Another important parameter that must be tuned to suit the properties of the particular film being produced is contact pressure.  Contact pressure is provided by the winding drum in surface winders and the lay on roll in center turret winders.  Some materials, notably tacky sealant layers (Ionomers, EVA, mLLDPE), benefit if there is a controlled layer of air entrapment wound into the roll between the successive layers of film.  This is achieved by running in gap winding mode.  Other materials can be relatively slippery (Polyamides, HDPE) and would not wind properly on a pure surface winder without the benefit of center assist driving the winding spindle.

Attention must also be paid to the cutover method with these films.  For cutting heavy gauge films with multiple layers of Nylon (typical thermoformable structures) Macro has developed a specially designed flying knife which will readily cut through multiple plies of 200µ - 300µ films.   

In summary, center-turret winders with precise control of lay on roll pressure and gap capability, or surface winders with center assist and gap winding, both styles including two-direction winding capability, will provide the film processor with the versatility to produce high quality wound rolls on the full range of materials utilized in the barrier film packaging market.